Mixed cation perovskite

ABSTRACT

The present invention relates to a crystalline compound comprising: (i) Cs+ (caesium); (ii) (H2N—C(H)═NH2)+ (formamidinium); (iii) one or more metal or metalloid dications [B]; and (iv) two or more different halide anions [X]. The invention also relates to a semiconductor device comprising a semiconducting material, which semiconducting material comprises the crystalline compound. The invention also relates to a process for producing a layer of the crystalline compound.

FIELD OF THE INVENTION

The present invention relates to a crystalline compound useful as asemiconductor material or a photoactive material. The invention alsorelates to a semiconductor device comprising a semiconducting material,which semiconducting material comprises the crystalline compound. Aprocess for producing a layer of the crystalline compound is alsodescribed.

The work leading to this invention has received funding from the EPSRCthrough the Supergen Solar Energy Hub SuperSolar (EP/M024881/1) and(EP/M014797/1) the ERC through the Stg-2011 HYPER and the US Office ofNaval Research (ONR).

BACKGROUND OF THE INVENTION

Terrestrial photovoltaic solar energy, predominantly based on singlejunction crystalline silicon, has a world record efficiency of 25% whichrepresents an upper limit on the efficiency which we expect commercialmodules eventually reach. To go beyond this performance, which isimportant in order to continuingly drive down the overall cost ofgenerating electricity from sun light, more advanced concepts arerequired (Shockley et al., J. Appl. Phys. 32, 510 (1961); Yin et al., J.Mater. Chem. A. 3, 8926-8942 (2014); and Polman et al., Nat. Mater. 11,174-7 (2012)). One such concept is to create a “tandem junction” byemploying a wide band gap “top cell” in combination with a silicon“bottom cell”, which could increase the realistically achievableefficiency to beyond 30% (Sivaram et al., Sci. Am. 313, 54-59 (2015)).For maximizing performance, a crystalline silicon (c-Si) bottom-cellwith a band gap of 1.1 eV requires a top-cell material with a band gapof 1.75 eV, in order to perfectly current-match both junctions (Shah etal., Science. 285, 692-699 (1999)). However, to date, there has yet tobe a suitable wide band gap top cell material for silicon or thin filmtechnologies, which offers stability, high performance, and low cost. Inrecent years, metal halide perovskite-based solar cells have gainedsignificant attention due to their high power conversion efficiencies(PCE) and low processing cost (C. R. Kagan, Science. 286, 945-947(1999); Lee et al., Science. 338, 643-7 (2012); Liu et al., Nature. 501,395-8 (2013); Burschka et al., Nature. 499, 316-9 (2013); Green et al.,Nat. Photonics. 8, 506-514 (2014); Jeon et al., Nat. Mater. 13, 1-7(2014); and Jeon et al., Nature. 517, 476-480 (2015)). An attractivefeature of this material is the ability to tune its band gap from 1.48to 2.3 eV (Noh et al., Nano Lett. 13, 1764-9 (2013) and Eperon et al.,Energy Environ. Sci. 7, 982 (2014)) implying that one could potentiallyfabricate an ideal material for tandem cell applications.

Perovskite-based solar cells are generally fabricated usingorganic-inorganic trihalide perovskites with the formulation ABX₃, whereA is the methylammonium (CH₃NH₃) (MA) or formamidinium (HC(NH₂)₂) (FA)cation, B is commonly lead (Pb), while X is a halide (Cl, Br, I).Although these perovskite structures offer high power conversionefficiencies (PCE), reaching over 20% PCE with band gaps of around 1.5eV, fundamental issues have been discovered when attempting to tunetheir band gaps to hit the optimum 1.7 to 1.8 eV range (Yang et al.,Science. 348, 1234-1237 (2015)). In the case of methylammonium leadtrihalide (MAPb(I_((1-y))Br_(y))₃), Hoke et al. (Chem. Sci. 6, 613-617(2014)) reported that light-soaking induces a halide segregation withinthe absorbing material. The formation of iodide-rich domains with lowerband gap results in an increase in sub-gap absorption and a red-shift ofphotoluminescence (PL). The lower band gap regions limit the voltageattainable with such a material, so this band gap “photo-instability”limits the use of MAPb(I_((1-y))Br_(y))₃ in tandem devices. In addition,when considering real-world applications, it has been shown that MAPbI₃is inherently thermally unstable at 85° C., even in an inert atmospherethis is the temperature that international regulations require acommercial PV product to be capable of withstanding.

Concerning the more thermally stable FAPbX₃ perovskite, open-circuitvoltage (V_(OC)) pinning has also been observed inFAPb(I_((1-x))Br_(x))₃ devices, where an increase in optical band gapdid not result in an expected increase in V_(OC). Furthermore, as theiodide is substituted with bromide, a crystal phase transition isobserved from a trigonal to a cubic structure; in compositions close tothe transition, the material appears unable to crystallize, resulting inan apparently “amorphous” phase with high levels of energetic disorderand unexpectedly low absorption. These compositions additionally havemuch lower charge-carrier mobilities in the range of 1 cm²/Vs, incomparison to over 20 cm²/Vs in the neat iodide perovskite, and higherrecombination rates than in the crystalline material. This is not anissue for high efficiency single junction solar cells since they can befabricated with the iodine rich, phase stable material, butdisadvantageously for tandem applications, this occurs right at the Brcomposition needed to form the desired top-cell band gap of 1.7 to 1.8eV.

Nevertheless, perovskite/silicon tandem solar cells have already beenreported in a 4-terminal and 2-terminal architectures (Bailie et al.,Energy Environ. Sci. 8, 956-963 (2015); Löper et al., Phys. Chem. Chem.Phys. 17, 1619-29 (2015); and Mailoa et al., Appl. Phys. Lett. 106,121105 (2015)). However, their reported efficiencies have yet to surpassthe optimized single-junction efficiencies, in part due to non-idealabsorber band gaps having been employed. To avoid the halide segregationproblem, it is possible to form a lower band gap triiodide perovskitematerial and current-match the top and bottom junctions in a monolithicarchitecture by simply reducing the thickness of the top-cell. However,this method results in a non-ideal efficiency.

Choi et al. (Nano Energy (2014) 7, 80-85) describes mixed cationperovskite compounds comprising both cesium and methylammonium. Lee etal. (Adv. Energy Mater. (2015) 5, 1501310) describes mixed cationperovskite compounds comprising both cesium and formamidinium. Pellet etal. (Angew. Chem. Int. Ed. (2014) 53, 3151-3157) describes mixed cationperovskite compounds comprising both methylammonium and formamidinium.Jeon et al. (Nature (2015) 517, 476-479) describes mixed cation/mixedhalide perovskite compounds comprising both methylammonium andformamidinium and both iodide and bromide. However, these perovskite donot yet provide the full tunability and stability required for use intandem cells.

There is a need to develop a new photoactive material which has atunable band gap and does not have the issues of halide segregation orthermal instability.

SUMMARY OF THE INVENTION

The inventors have discovered a perovskite composition which deliversthe ideal absorber material for stable and efficient hybrid tandem solarcells. In particular, the inventors have addressed the issues of forminga photo-stable FA based perovskite with the ideal band gap for tandemapplications by partially substituting the formamidinium cation withcaesium. The remarkable observation has been made that the phaseinstability region is then entirely eliminated in the iodide to bromidecompositional range.

The invention therefore provides a crystalline compound comprising:

-   -   (i) Cs⁺,    -   (ii) (H₂N—C(H)═NH₂)⁺;    -   (iii) one or more metal or metalloid dications [B]; and    -   (iv) two or more different halide anions [X].

The invention also provides a semiconducting material comprising thecrystalline compound of the invention.

The invention also provides a semiconductor device comprising asemiconducting material, which semiconducting material comprises acrystalline compound, which crystalline compound comprises:

-   -   (i) Cs⁺;    -   (ii) (H₂N—C(H)═NH₂)⁺;    -   (iii) one or more metal or metalloid dications [B]; and    -   (iv) two or more different halide anions [X].

Further provided by the invention is a process for producing a layer ofa crystalline compound, which crystalline compound comprises:

-   -   (i) Cs⁺;    -   (ii) (H₂N—C(H)═NH₂)⁺;    -   (iii) one or more metal or metalloid dications [B]; and    -   (iv) a first halide anion X; and    -   (v) a second halide anion X′,    -   which process comprises:    -   (a) disposing on a substrate a precursor composition comprising:        -   CsX and/or CsX′;        -   (H₂N—C(H)═NH₂)X and/or (H₂N—C(H)═NH₂)X′;        -   BX₂ and/or BX′₂.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows (A) a photograph of perovskite films with Br compositionincreasing from X=0 to 1 for the FAPb(I_((1-x))Br_(x))₃ system and (B)for the FA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃ system.

FIG. 2 shows (top) UV-Vis absorbance spectra of perovskite films for theFAPb(I_((1-x))Br_(x))₃ system and (bottom) for theFA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃ system.

FIG. 3 shows (top) the x-ray diffraction (XRD) pattern ofFAPb(I_((1-x))Br_(x))₃ perovskites and (bottom) of theFA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃ system.

FIG. 4 shows (A) normalized photoluminescence (PL) measurement of theMAPb(I_(0.6)Br_(0.4))₃ thin film, measured after 0, 5, 15, 30 and 60minutes of light exposure using a power density of 3 mW cm′ and awavelength of 550 nm as excitation source and (B) PL Measurement of theFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ thin film exposed to identicallight illumination conditions.

FIG. 5 shows semi-log plot of external quantum efficiency (EQE) at theabsorption onset for the same sample as FIG. 4, measured using FTPS inshort-circuit (Jsc) configuration.

FIG. 6 shows OPTP transients for aFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ thin film, measured followingexcitation with a 35 fs light pulse of wavelength 400 nm with differentfluences.

FIG. 7 shows charge-carrier diffusion length L as a function of chargeconcentration for the perovskite.

FIG. 8 shows a SEM image of cross-section of a planar heterojunctionsolar cell according to the invention.

FIG. 9 shows forward bias to short-circuit J-V curve for the bestperovskite devices fabricated, using a SnO₂/PCBM compact layer as thehole-blocking layer and Spiro-OMeTAD, as the electron-blocking holecollection layer with either a Ag metal, or semi-transparent ITO topelectrode, measured at 0.38V/s scan rate.

FIG. 10 shows photocurrent density and power conversion efficiencymeasured at maximum power point for a 30 s timespan.

FIG. 11 shows external quantum efficiency (EQE) spectrum measured inshort-circuit (Jsc) configuration for the highest efficiency perovskitecell and the SHJ cell measured with the simulated sun light filteredthrough the semi-transparent perovskite cell.

FIG. 12 shows x-ray diffraction pattern (XRD) for the entire range ofFAPb(I_((1-x))Br_((x)))₃ perovskites formed on fluorine-doped tin oxide(FTO) coated glass substrates. Peaks labelled with # are assigned to theFTO substrate.

FIG. 13 shows x-ray diffraction pattern (XRD) for the entire range ofFA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_((x)))₃ perovskites formed onfluorine-doped tin oxide (FTO) coated glass substrates. Peaks labelledwith # are assigned to the FTO substrate.

FIG. 14 shows a photograph of perovskite films with a Cs compositionincreasing from x=0 to 0.2.

FIG. 15 shows the UV-Vis absorbance of perovskite films where the ratiox of Cs is varied from 0 to 0.5.

FIG. 16 shows steady-state photoluminescence spectra for perovskiteshaving a Cs content varying from x=0.1 to 0.5.

FIG. 17 shows x-ray diffraction (XRD) pattern of perovskite materialshowing the formation of a single crystalline (100) cubic peak and itsshift with increasing Cs content from x=0 to 0.75.

FIG. 18 shows the variation in band gap with cubic lattice parameter asdetermined from XRD pattern and a Tauc plot as the Cs content x variesfrom 0.1 to 0.5.

FIG. 19 shows a Tauc plot of FA_((1-x))Cs_(x)Pb(I_(0.6)Br_(0.4))₃ as theCs content x varies from 0 to 0.5 assuming direct band gap and showingdetermination of estimated band gap from intercept.

FIG. 20 shows the x-ray diffraction pattern (XRD) for the full range ofFA_((1-x))Cs_((x))Pb(I_(0.6)Br_(0.4))₃ perovskites formed onfluorine-doped tin oxide (FTO) coated glass substrates when annealed at170° C. for 10 minutes, using a 0.55M solution. Peaks labelled with #are assigned to the FTO substrate.

FIG. 21 shows the lattice constant ofFA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_((x)))₃ perovskite system plotted as afunction of bromide content. This linear relationship between latticeconstant and bromide content indicates that this system follows Vegard'slaw

FIG. 22 shows Tauc plot of FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃assuming direct band gap and showing determination of estimated band gapfrom intercept.

FIG. 23 shows photoluminescence spectra ofFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ following excitation with pulsefluence of 0.5 μJcm⁻² and laser intensity of 5 Wcm⁻² at a wavelength of405 nm.

FIG. 24 shows time-resolved PL spectroscopy ofFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ following excitation at 400 nmwith an excitation fluence of 0.49 μJ cm⁻², measured under vacuum.

FIG. 25 shows (top) Power conversion efficiency (PCE) of compositionranging from x=0 to 0.5 of Cs and (bottom) Fill Factor (FF) ofcomposition ranging from x=0 to 0.5 of Cs.

FIG. 26 shows the PCE of devices containing perovskites having acomposition ranging from x=0 to 0.2 of Cs.

FIG. 27 shows J-V characteristics of a device having aFTO/SnO₂/PCBM/perovskite/Spiro-OMeTAD/Ag architecture under simulatedair-mass (AM) 1.5 100 mW cm⁻² sun light using a 0.38V/s scan rate andphotocurrent density and power conversion efficiency as a function oftime held at maximum power voltage. The perovskite was obtained from theoptimized precursor solution composition ofFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃.

FIG. 28 shows photocurrent density and power conversion efficiencymeasured over 1 minute for the device of FIG. 27.

FIG. 29 shows J-V characteristics ofFTO/SnO₂/PCBM/perovskite/Spiro-OMeTAD/Ag architecture measured using anactive area and masked aperture of 0.715 cm², under simulated air-mass(AM) 1.5, 109 mW cm⁻² sun light using a 0.38V/s scan rate andphotocurrent density and power conversion efficiency as a function oftime held at maximum power voltage obtained for the optimized precursorsolution composition of FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃.

FIG. 30 shows photocurrent density and power conversion efficiencymeasured over 1 minute for the device of FIG. 29.

FIG. 31 shows the calculated photon flux of black body at 300K comparedto AM1.5 solar spectrum photon flux.

FIG. 32 shows current-voltage characteristics of unfiltered and filteredSHJ cell under simulated air-mass (AM) 1.5 100 mW cm⁻² sun light. TheSHJ cell was filtered using a FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃with semi-transparent ITO rear-contact.

FIG. 33 shows device performance of a series ofFA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃ with various I/Br compositions,and in particular power conversion efficiency (PCE) of compositionranging from x=0 to 0.4 of Br.

FIG. 34 shows the normalized electroluminescence spectra of the seriesof FA_(0.825)Cs_(0.175)Pb(I_((1-x))Br_(x))₃ devices with x=[0, 0.05,0.1, 0.2, 0.3, 0.4].

FIG. 35 shows the electroluminescence spectra ofFA_(0.825)Cs_(0.175)Pb(I_((1-x))Br_(x))₃ with x=0.3 as a function ofapplied voltage.

FIG. 36 shows IV and LV performance for the series ofFA_(0.825)Cs_(0.175)Pb(I_((1-x))Br_(x))₃ with x=[0, 0.05, 0.1, 0.2, 0.3,0.4]. The cell area was 0.909 cm².

FIG. 37 shows the comparison of the stability of devices comprising themixed-cation perovskite according to the invention and a methylammoniumlead halide perovskite. In the figure legend, “doped” refers to a cellcontaining an n-doped C₆₀ n-type electron collection layer, “control”refers to a cell containing a neat C₆₀ n-type electron collection layer,without additional n-doping.

FIG. 38 shows the UV-Vis absorbance spectra for the visible range ofFA_(0.83)Cs_(0.17)Pb(Cl_(X)Br_(Y)I_(Z))₃ perovskites formed onfluorine-doped tin oxide (FTO) coated glass substrates.

FIG. 39 shows the normalized photoluminescence spectra for the visiblerange of FA_(0.83)Cs_(0.17)Pb(Cl_(X)Br_(Y)I_(Z))₃ perovskites formed onfluorine-doped tin oxide (FTO) coated glass substrates.

FIG. 40 shows the x-ray diffraction pattern (XRD) for the visible rangeof FA_(0.83)Cs_(0.17)Pb(Cl_(X)Br_(Y)I_(Z))₃ perovskites formed onfluorine-doped tin oxide (FTO) coated glass substrates.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a crystalline compound comprising: (i) Cs⁺, (ii)(H₂N—C(H)═NH₂)⁺; (iii) one or more metal or metalloid dications [B]; and(iv) two or more different halide anions [X]. Thus, the crystallinecompound is a crystalline compound having a formula which comprises Cs⁺,(H₂N—C(H)═NH₂)⁺, [B] and [X]. For instance, the crystalline compound maybe a compound of formula Cs_(p)(H₂N—C(H)═NH₂)_(q)[B]_(r)[X]_(s) where p,q, r and s are numbers from 0.01 to 8.0. The crystalline compound may insome cases comprise additional ions.

A crystalline compound is a compound defined by a formula comprising twoor more ions, wherein the ions in the compound are arranged in anextended crystal structure. For instance, sodium chloride (NaCl) is acrystalline compound, the formula of which comprises Na⁺ and Cl⁻, andwhere these two ions are arranged in an extended crystal structure (therock salt structure). Bonding between atoms and/or ions within acrystalline compound is typically intermediate between fully ionic andfully covalent bonding, although there is a predominance of ionicbonding.

An amount of a crystalline compound typically comprises crystallites ofthe crystalline compound, i.e. a plurality of single crystal domains ofthe crystalline compound, which may be in the form of a powder or asolid. In a solid comprising a plurality of crystallites of acrystalline compound (which may be referred to as a polycrystallinesolid), grain boundaries will be present between adjacent crystalliteswhere there is a discontinuity in the extended crystal structure. Acrystalline compound may be in the form of a single crystal.

The halide anions [X] may be selected from I⁻, Br⁻, Cl⁻ and F⁻.Typically, the two or more different halide anions [X] are selected fromI⁻, Br⁻ and Cl⁻. Preferably, the two or more different halide anions [X]are I⁻ and Br⁻. In that case, the crystalline compound comprises both I⁻and Br⁻. Alternatively, the two or more different halide anions [X] maybe Br⁻ and Cl⁻. The two or more different halide anions may be presentin any proportion. For instance, [X]₃ may correspond to (I, Br)₃ where Iand Br are in any proportion, or a proportion as further defined below.The two or different halide anions will all occupy X sites within thestructure of the crystalline compound. The halide anions may be orderedor disordered within those X sites.

The metal or metalloid dications may be dications derived from any metalin groups 1 to 16 of the periodic table of the elements or any of themetalloids. Metalloids are usually taken to be following elements: B,Si, Ge, As, Sb, Te and Po. Typically, the one or more metal or metalloiddications are selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺,Co²⁺, Pd²⁺, Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺. Preferably, the one or moremetal or metalloid dications [B] are selected from Pb²⁺, Ge²⁺ and Cu²⁺.More preferably, the one or more metal or metalloid dications [B] arePb²⁺, i.e. the formula of the crystalline compound comprises only Pb²⁺as the metal or metalloid dication.

Typically, the crystalline compound is a perovskite compound having athree dimensional perovskite structure and comprising (i) Cs⁺, (ii)(H₂N—C(H)═NH₂)⁺; (iii) one or more metal or metalloid dications [B]; and(iv) two or more different halide anions [X]. Thus, the crystallinecompound typically has the three-dimensional crystal structure which isrelated to that of CaTiO₃. The structure of CaTiO₃ can be represented bythe formula ABX₃, wherein A and B are cations of different sizes and Xis an anion. In the unit cell for a perfect ABX₃ structure, the Acations are at (0,0,0), the B cations are at (½, ½, ½) and the X anionsare at (½, ½, 0). The A cation is usually larger than the B cation. Theskilled person will appreciate that when A, B and X are varied, thedifferent ion sizes may cause the structure of the perovskite materialto distort away from the structure adopted by CaTiO₃ to a lower-symmetrydistorted structure. Such distorted structures still comprise a threedimensional array of BX₆ octahedra and are still examples of the threedimensional perovskite structure. The skilled person will appreciatethat the perovskite compound can be represented by the formula[A][B][X]₃, wherein [A] is at least one cation, [B] is at least onecation and [X] is at least one anion. When the perovskite comprises morethan one A cation (e.g. wherein [A] is (A¹, A²) or (A¹, A², A³)), thedifferent A cations may distributed over the A sites in an ordered ordisordered way. When the perovskite comprises more than one B cation,the different B cations may distributed over the B sites in an orderedor disordered way. When the perovskite comprise more than one X anion,the different X anions may distributed over the X sites in an ordered ordisordered way. The symmetry of a perovskite comprising more than one Acation, more than one B cation or more than one X cation, will be lowerthan that of CaTiO₃.

The crystalline compound is often a perovskite compound of formula (I):

Cs_(x)(H₂N—C(H)═NH₂)_((1-x))[B][X]₃  (I);

wherein: [B] is the one or more metal or metalloid dications; [X] is thetwo or more different halide anions; and x is from 0.01 to 0.99.

Preferably, the crystalline compound is a perovskite compound of formula(II):

Cs_(x)(H₂N—C(H)═NH₂)_((1-x))[B]X_(3y)X′_(3(1-y))  (II);

wherein: [B] is the one or more metal or metalloid dications; X is afirst halide anion selected from I⁻, Br⁻, Cl⁻ and F⁻; X′ is a secondhalide anion which is different from the first halide anion and isselected from I⁻, Br⁻, Cl⁻ and F⁻; x is from 0.01 to 0.99; and y is from0.01 to 0.99. Typically X is a first halide anion selected from I⁻, Br⁻and Cl⁻ and X′ is a second halide anion which is different from thefirst halide anion and is selected from I⁻, Br⁻ and Cl⁻.

Preferably, in formula (II), [B] is Pb²⁺ and the crystalline compound isof formula Cs_(x)(H₂N—C(H)NH₂)_((1-x))PbX_(3y)X′_(3(1-y)).

Preferably, X is Br⁻ and X′ is I⁻. Alternatively, X is Cl⁻ and X′ is I⁻,or X is Cl⁻ and X′ is Br⁻.

Typically, in formula (I), (II) or (III), x is from 0.05 to 0.50.Preferably, x is from 0.10 to 0.30. More preferably, x is from 0.15 to0.20.

Typically, in formula (II) or (III), y is from 0.01 to 0.70. Preferably,y is from 0.20 to 0.60. More preferably is from 0.30 to 0.50.

For instance, in formula (II) or (III), x may be from 0.10 to 0.30 and ymay be from 0.20 to 0.60.

Preferably, the crystalline compound is a perovskite compound of formula(III):

Cs_(x)(H₂N—C(H)═NH₂)_((1-x))PbBr_(3y)I_(3(1-y))  (III);

wherein: x is from 0.15 to 0.20; and y is from 0.30 to 0.50.

Often, the crystalline compound is of formulaCs_(0.2)(H₂N—C(H)═NH₂)_(0.8)Pb(Br_(0.4)I_(0.6))₃, where the proportionof each ion is to 1 decimal place. As such, the proportion of Cs⁺ may befrom 0.150 to 0.249, the proportion of (H₂N—C(H)═NH₂)⁺ may be from 0.750to 0.849, the proportion of Br⁻ may be from 0.350 to 0.449 and theproportion of I⁻ may be from 0.50 to 0.649. The proportion of r may befrom 0.550 to 0.649.

The crystalline compound may be a compound of formulaCs_(0.2)(H₂N—C(H)═NH₂)_(0.8)Pb(Br_(0.4)I_(0.6))₃. In one embodiment, thecrystalline compound isCs_(0.17)(H₂N—C(H)═NH₂)_(0.83)Pb(Br_(0.4)I_(0.6))₃. For instance, thecrystalline compound may beCs_(0.175)(H₂N—C(H)NH₂)_(0.825)Pb(Br_(0.4)I_(0.6))₃.

As the skilled person will appreciate, there may be some minorvariations in the stoichiometry of the crystalline compound, forinstance within different regions of a solid form of the crystallinecompound. However, such solid forms having some variation in thestoichiometry of the crystalline compound will still contain an amountof the specific crystalline compounds described herein. Typically, acrystalline compound according to the invention may have an averagestoichiometry corresponding to the stoichiometry as set out in aformulae described herein. For instance, a solid form of a crystallinecompound according to the invention may comprise from 15 mol % to 20 mol% of Cs relative to the amount of Pb and from 85 mol % to 80 mol % ofH₂N—C(H)═NH₂ relative to the amount of Pb.

The crystalline compound according to the invention has a fully tunableband gap and as such is well suited to use as a photoactive material intandem with a second photoactive material such as silicon. Thecrystalline compound typically has a band gap of from 1.25 to 2.25 eV.Preferably, the crystalline compound has a band gap of from 1.5 to 2.0eV. The crystalline compound may, for instance, have a band gap of from1.7 to 1.8 eV. The band gap can be determined by making a Tauc plot, asdescribed in Tauc, J., Grigorovici, R. & Vancu, a. Optical Propertiesand Electronic Structure of Amorphous Germanium. Phys. Status Solidi 15,627-637 (1966) where the square of the product of absorption coefficienttimes photon energy is plotted on the y-axis against photon energy onthe x-axis with the straight line intercept of the absorption edge withthe x-axis giving the optical band gap of the semiconductor (forinstance as shown in FIG. 22). The band gap is typically as measured at20° C.

The crystalline compound may be in the form of a solid layer of thecrystalline compound, for instance a layer without open porosity. Thecrystalline compound may be in the form of a compact layer of thecrystalline compound.

The term “porous”, as used herein, refers to a material within whichpores are arranged. Thus, for instance, in a porous scaffold materialthe pores are volumes within the scaffold where there is no scaffoldmaterial. The individual pores may be the same size or different sizes.The size of the pores is defined as the “pore size”. The limiting sizeof a pore, for most phenomena in which porous solids are involved, isthat of its smallest dimension which, in the absence of any furtherprecision, is referred to as the width of the pore (i.e. the width of aslit-shaped pore, the diameter of a cylindrical or spherical pore,etc.). To avoid a misleading change in scale when comparing cylindricaland slit-shaped pores, one should use the diameter of a cylindrical pore(rather than its length) as its “pore-width” (J. Rouquerol et al.,“Recommendations for the Characterization of Porous Solids”, Pure &Appl. Chem., Vol. 66, No. 8, pp. 1739-1′758, 1994). The followingdistinctions and definitions were adopted in previous IUPAC documents(K. S. W. Sing, et al, Pure and Appl. Chem., vol 0.57, n04, pp 603-919,1985; and IUPAC “Manual on Catalyst Characterization”, J. Haber, Pureand Appl. Chem., vol 0.63, pp. 1227-1246, 1991): micropores have widths(i.e. pore sizes) smaller than 2 nm; Mesopores have widths (i.e. poresizes) of from 2 nm to 50 nm; and Macropores have widths (i.e. poresizes) of greater than 50 nm. In addition, nanopores may be consideredto have widths (i.e. pore sizes) of less than 1 nm.

Pores in a material may include “closed” pores as well as open pores. Aclosed pore is a pore in a material which is a non-connected cavity,i.e. a pore which is isolated within the material and not connected toany other pore and which cannot therefore be accessed by a fluid (e.g. aliquid, such as a solution) to which the material is exposed. An “openpore” on the other hand, would be accessible by such a fluid. Theconcepts of open and closed porosity are discussed in detail in J.Rouquerol et al., “Recommendations for the Characterization of PorousSolids”, Pure & Appl. Chem., Vol. 66, No. 8, pp. 1739-1′758, 1994.

Open porosity, therefore, refers to the fraction of the total volume ofthe porous material in which fluid flow could effectively take place. Ittherefore excludes closed pores. The term “open porosity” isinterchangeable with the terms “connected porosity” and “effectiveporosity”, and in the art is commonly reduced simply to “porosity”.

The term “without open porosity”, as used herein, therefore refers to amaterial with no effective open porosity. Thus, a material without openporosity typically has no macropores and no mesopores. A materialwithout open porosity may comprise micropores and nanopores, however.Such micropores and nanopores are typically too small to have a negativeeffect on a material for which low porosity is desired.

The term “compact layer”, as used herein, refers to a layer withoutmesoporosity or macroporosity. A compact layer may sometimes havemicroporosity or nanoporosity.

The invention also provides particles comprising the crystallinecompound. Thus, the crystalline compound may be in the form of particlescomprising the crystalline compound. Typically, such particles have anumber average particle size of less than or equal to 1.0 mm. Theparticle size of a particle is the diameter of a sphere having the samevolume as the particle. This may be measured by laser diffraction.

The particles may be microparticles, mesoparticles or nanoparticles.Microparticles are particles having a number average particle size offrom 1.0×10⁻⁷ to 1.0×10⁻⁴ m (i.e. from 0.1 to 100 μm). Mesoparticles areparticles having a number average particle size of from 1.0×10⁻⁸ to1.0×10⁻⁶ m (i.e. from 10 to 1000 nm). Nanoparticles are particles havinga number average particle size of from 1.0×10⁻⁹ to 1.0×10⁻⁷ m (i.e. from1.0 to 100 nm).

The particles of the crystalline compound may be present in a colloid.Such a colloid may comprise a continuous liquid or solid phase and,disposed in the continuous phase, a plurality of the particles of thecrystalline compound.

The invention also provides a semiconducting material comprising acrystalline compound as defined herein. The semiconducting material ispreferably a photoactive material.

The term “semiconductor” or “semiconducting material”, as used herein,refers to a material with electrical conductivity intermediate inmagnitude between that of a conductor and a dielectric. A semiconductormay be an negative (n)-type semiconductor, a positive (p)-typesemiconductor or an intrinsic (i) semiconductor. Examples ofsemiconducting materials include photoactive materials. The term“photoactive material”, as used herein, refers to a material whicheither (i) absorbs light, which may then generate free charge carriers;or (ii) accepts charge, both electrons and holes, which may subsequentlyrecombine and emit light. A photoabsorbent material is a material whichabsorbs light, which may then generate free charge carriers (e.gelectrons and holes). A “photoemissive material” is a material whichabsorbs light of energies higher than band gap and reemits light atenergies at the band gap.

The semiconducting material typically comprises greater than or equal to80% by weight of the crystalline compound. The semiconducting materialmay for instance comprise greater than or equal to 90% by weight of thecrystalline compound.

The semiconducting material may be in the form of a layer, for instancea compact layer of the semiconducting material.

The invention also provides a semiconductor device comprising asemiconducting material, which semiconducting material comprises acrystalline compound, which crystalline compound comprises: (i) Cs⁺,(ii) (H₂N—C(H)═NH₂)⁺; (iii) one or more metal or metalloid dications[B]; and (iv) two or more different halide anions [X]. The crystallinecompound may be as defined herein, for instance a compound of formula(III).

The term “semiconductor device”, as used herein, refers to a devicecomprising a functional component which functional component comprises asemiconductor material. This term may be understood to be synonymouswith the term “semiconducting device”. Examples of semiconductor devicesinclude an optoelectronic device, a photovoltaic device, a solar cell, aphoto detector, a photodiode, a photosensor, a chromogenic device, atransistor, a light-sensitive transistor, a phototransistor, a solidstate triode, a battery, a battery electrode, a capacitor, asuper-capacitor, a light-emitting device and a light-emitting diode. Theterm “optoelectronic device”, as used herein, refers to devices whichsource, control, detect or emit light. Light is understood to includeany electromagnetic radiation. Examples of optoelectronic devicesinclude photovoltaic devices, photodiodes (including solar cells),phototransistors, photomultipliers, photoresistors, light emittingdevices, light emitting diodes, lasers and charge injection lasers.

The semiconductor device is typically an optoelectronic device.Preferably, the semiconductor device is a photovoltaic device, aphotodetector or a light-emitting device. More preferably, thesemiconductor device is a photovoltaic device (for instance a solarcell).

The semiconductor device typically comprises a layer of thesemiconducting material. The layer of said semiconducting material mayfor instance have a thickness of from 5 nm to 10000 nm. Typically, thesemiconductor device comprises a layer of the semiconducting material,which layer preferably has a thickness of from 5 nm to 1000 nm.Preferably, the layer of the semiconducting material has a thickness offrom 100 nm to 700 nm, for instance from 200 nm to 500 nm. The layer ofthe semiconducting material may consist, or consist essentially of (e.g.greater than or equal to 99 wt %) a layer of the compound having athickness of from 100 nm to 700 nm. In some devices, the layer may be athin sensitising layer, for instance having a thickness of from 5 nm to50 nm. In devices wherein the layer of said semiconducting materialforms a planar heterojunction with an n-type or p-type region, the layerof said photoactive material may have a thickness of greater than orequal to 100 nm. Preferably, the layer of said photoactive material hasa thickness of from 100 nm to 700 nm, for instance from 200 nm to 500nm.

Typically, the semiconductor device comprises: an n-type regioncomprising at least one n-type layer; a p-type region comprising atleast one p-type layer; and, disposed between the n-type region and thep-type region: a layer of said semiconducting material. An n-type layeris typically a layer of an n-type semiconductor. A p-type layer istypically a layer of a p-type semiconductor.

The n-type region comprises at least one n-type layer. The n-type regionmay alternatively comprise an n-type layer and an n-type excitonblocking layer. Such an n-type exciton blocking layer is typicallydisposed between the n-type layer and the layer(s) comprising thesemiconducting material. The n-type region may have a thickness of from50 nm to 1000 nm. For instance, the n-type region may have a thicknessof from 50 nm to 500 nm, or from 100 nm to 500 nm.

Preferably, the n-type region comprises a compact layer of an n-typesemiconductor.

The n-type semiconductor may be selected from a metal oxide, a metalsulphide, a metal selenide, a metal telluride, a perovskite, amorphousSi, an n-type group IV semiconductor, an n-type group III-Vsemiconductor, an n-type group II-VI semiconductor, an n-type groupI-VII semiconductor, an n-type group IV-VI semiconductor, an n-typegroup V-VI semiconductor, and an n-type group II-V semiconductor, any ofwhich may be doped or undoped. Typically, the n-type semiconductor isselected from a metal oxide, a metal sulphide, a metal selenide, and ametal telluride. For instance, the n-type region may comprise aninorganic material selected from oxide of titanium, tin, zinc, niobium,tantalum, tungsten, indium, gallium, neodymium, palladium, or cadmium,or an oxide of a mixture of two or more of said metals. For instance,the n-type layer may comprise TiO₂, SnO₂, ZnO, Nb₂O₅, Ta₂O₅, WO₃, W₂O₅,In₂O₃, Ga₂O₃, Nd₂O₃, PbO, or CdO.

Typically, the n-type region comprises SnO₂ or TiO₂, for instance acompact layer of TiO₂ or SnO₂. Often, the n-type region also comprises alayer of a fullerene or a fullerene derivative (for instance C₆₀ orPhenyl-C₆₁-butyric acid methyl ester (PCBM)).

The p-type region comprises at least one p-type layer. The p-type regionmay alternatively comprise an p-type layer and a p-type exciton blockinglayer. Such a p-type exciton blocking layer is typically disposedbetween the p-type layer and the layer(s) comprising the semiconductingmaterial. The p-type region may have a thickness of from 50 nm to 1000nm. For instance, the p-type region may have a thickness of from 50 nmto 500 nm, or from 100 nm to 500 nm.

Preferably, the p-type region comprises a compact layer of a p-typesemiconductor

Suitable p-type semiconductors may be selected from polymeric ormolecular hole transporters. The p-type layer employed in thesemiconductor device of the invention may for instance comprisespiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene)),P3HT (poly(3-hexylthiophene)), PCPDTBT(Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]]),PVK (poly(N-vinylcarbazole)), HTM-TFSI (1-hexyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide), Li-TF SI (lithiumbis(trifluoromethanesulfonyl)imide) or tBP (tert-butylpyridine). Thep-type region may comprise carbon nanotubes. Usually, the p-typematerial is selected from spiro-OMeTAD, P3HT, PCPDTBT and PVK.Preferably, the p-type layer employed in the optoelectronic device ofthe invention comprises spiro-OMeTAD.

In some embodiments, the p-type layer may comprise an inorganic holetransporter. For instance, the p-type layer may comprise an inorganichole transporter comprising an oxide of nickel, vanadium, copper ormolybdenum; CuI, CuBr, CuSCN, Cu₂O, CuO or CIS; a perovskite; amorphousSi; a p-type group IV semiconductor, a p-type group III-V semiconductor,a p-type group II-VI semiconductor, a p-type group I-VII semiconductor,a p-type group IV-VI semiconductor, a p-type group V-VI semiconductor,and a p-type group II-V semiconductor, which inorganic material may bedoped or undoped. The p-type layer may be a compact layer of saidinorganic hole transporter.

The layer of the semiconducting material typically forms a planarheterojunction with the n-type region or the p-type region. The layer ofthe semiconducting material typically forms a first planarheterojunction with the n-type region and a second planar heterojunctionwith the p-type region. This forms a planar heterojunction device. Theterm “planar heterojunction” as used herein refers to a junction betweentwo regions where one region does not infiltrate the other. This doesnot require that the junction is completely smooth, just that one regiondoes not substantially infiltrate pores in the other region.

In some embodiments, it is desirable to have a porous scaffold materialpresent. The layer of a porous scaffold is usually in contact with acompact layer of a semiconductor material, for instance an n-typecompact layer or a p-type compact layer. The layer of a porous scaffoldis usually also in contact with the semiconducting material. Thescaffold material is typically mesoporous or macroporous. The scaffoldmaterial may aid charge transport from the semiconducting material to anadjacent region. The scaffold material may also, or alternatively, aidformation of the layer of the semiconducting material during deviceconstruction. The porous scaffold material is typically infiltrated bythe semiconducting material.

Thus, in some embodiments, the semiconductor device comprises:

-   -   an n-type region comprising at least one n-type layer;    -   a p-type region comprising at least one p-type layer; and,        disposed between the n-type region and the p-type region:    -   (i) a porous scaffold material; and    -   (ii) said semiconducting material in contact with the scaffold        material.

Typically, the semiconducting material in the first layer is disposed inpores of the scaffold material. The scaffold material is typicallymesoporous. The scaffold material may be macroporous.

Typically, the porous scaffold material comprises a dielectric materialor a charge-transporting material. The scaffold material may be adielectric scaffold material. The scaffold material may be acharge-transporting scaffold material. The porous scaffold material maybe an electron-transporting material or a hole-transporting scaffoldmaterial. n-type semiconducting materials are examples ofelectron-transporting materials. p-type semiconductors are examples ofhole-transporting scaffold materials. Preferably, the porous scaffoldmaterial is a dielectric scaffold material or an electron-transportingscaffold material (e.g. an n-type scaffold material). The porousscaffold material may be a charge-transporting scaffold material (e.g.an electron-transporting material such as titania, or alternatively ahole transporting material) or a dielectric material, such as alumina.The term “dielectric material”, as used herein, refers to material whichis an electrical insulator or a very poor conductor of electric current.The term dielectric therefore excludes semiconducting materials such astitania. The term dielectric, as used herein, typically refers tomaterials having a band gap of equal to or greater than 4.0 eV. (Theband gap of titania is about 3.2 eV.)

The porous scaffold material typically comprises an n-type semiconductoror a dielectric material. For instance, the device may comprise a layerof said porous scaffold material, where the porous scaffold materialcomprises an n-type semiconductor.

The porous scaffold is typically in the form of a layer. For instance,the porous scaffold may be a layer of porous scaffold material,typically having a thickness of from 5 nm to 500 nm, for instance from10 nm to 200 nm.

In some embodiments, the semiconductor device comprises:

-   -   an n-type region comprising at least one n-type layer;    -   a p-type region comprising at least one p-type layer; and,        disposed between the n-type region and the p-type region:    -   (i) a first layer which comprises a porous scaffold material and        said semiconducting material; and    -   (ii) a capping layer disposed on said first layer, which capping        layer is a layer of said semiconducting material without open        porosity,    -   wherein the semiconducting material in the capping layer is in        contact with the semiconducting material in the first layer.

In some embodiments, the scaffold material is porous and thesemiconducting material in the first layer is disposed in pores of thescaffold material. The effective porosity of said scaffold material isusually at least 50%. For instance, the effective porosity may be about70%. In one embodiment, the effective porosity is at least 60%, forinstance at least 70%.

Typically, the semiconducting material (or photoactive material) in thefirst layer contacts one of the p-type and n-type regions, and thesemiconducting material in the capping layer contacts the other of thep-type and n-type regions. The semiconducting material in the cappinglayer typically forms a planar heterojunction with the p-type region orthe n-type region.

In one embodiment, the semiconducting material in the capping layercontacts the p-type region, and the semiconducting material in the firstlayer contacts the n-type region. In another embodiment, thesemiconducting material in the capping layer contacts the n-type region,and the semiconducting material in the first layer contacts the p-typeregion (for instance in an inverted device).

The thickness of the capping layer is usually greater than the thicknessof the first layer. The majority of any photoactivity (e.g. lightabsorption or light emission) therefore usually occurs in a cappinglayer.

The thickness of the capping layer is typically from 10 nm to 100 um.More typically, the thickness of the capping layer is from 10 nm to 10um. Preferably, the thickness of the capping layer is from 50 nm to 1000nm, or for instance from 100 nm to 700 nm. The thickness of the cappinglayer may be greater than or equal to 100 nm.

The thickness of the first layer, on the other hand, is often from 5 nmto 1000 nm. More typically, it is from 5 nm to 500 nm, or for instancefrom 30 nm to 200 nm.

The semiconductor device typically further comprises one or more firstelectrodes and one or more second electrodes. The one or more firstelectrodes are typically in contact with the n-type region, if such aregion is present. The one or more second electrodes are typically incontact with the p-type region, if such a region is present. Typically:the one or more first electrodes are in contact with the n-type regionand the one or more second electrodes are in contact with the p-typeregion; or the one or more first electrodes are in contact with thep-type region and the one or more second electrodes are in contact withthe n-type region.

The first and second electrode may comprise any suitable electricallyconductive material. The first electrode typically comprises atransparent conducting oxide. The second electrode typically comprisesone or more metals. The second electrode may alternatively comprisegraphite. Typically, the first electrode typically comprises atransparent conducting oxide and the second electrode typicallycomprises one or more metals.

The transparent conducting oxide may be as defined above and is oftenFTO, ITO, or AZO, and typically ITO. The metal may be any metal.Generally the second electrode comprises a metal selected from silver,gold, copper, aluminium, platinum, palladium, or tungsten. Theelectrodes may form a single layer or may be patterned.

A semiconductor device according to the invention, for instance asensitized solar cell, may comprise the following layers in thefollowing order:

-   -   I. one or more first electrodes as defined herein;    -   II. optionally a compact n-type layer as defined herein;    -   III. a porous layer of an n-type material as defined herein;    -   IV. a layer of said semiconducting material (e.g. as a        sensitizer);    -   V. a p-type region as defined herein;    -   VI. optionally a further compact p-type layer as defined herein;        and    -   VII. one or more second electrodes as defined herein.

A semiconductor device according to the invention which is aphotovoltaic device may comprise the following layers in the followingorder:

-   -   I. one or more first electrodes as defined herein;    -   II. an n-type region comprising at least one n-type layer as        defined herein;    -   III. a layer of the semiconducting material comprising the        crystalline compound as defined herein;    -   IV. a p-type region comprising at least one p-type layer as        defined herein; and    -   V. one or more second electrodes as defined herein.

A photovoltaic device according to the invention may comprise thefollowing layers in the following order:

-   -   I. one or more first electrodes which comprise a transparent        conducting oxide, preferably FTO;    -   II. an n-type region comprising at least one n-type layer as        defined herein;    -   III. a layer of the semiconducting material as defined herein;    -   IV. a p-type region comprising at least one p-type layer as        defined herein; and    -   V. one or more second electrodes which comprise a metal,        preferably silver or gold.

The one or more first electrodes may have a thickness of from 100 nm to700 nm, for instance of from 100 nm to 400 nm. The one or more secondelectrodes may have a thickness of from 10 nm to 500 nm, for instancefrom 50 nm to 200 nm or from 10 nm to 50 nm. The n-type region may havea thickness of from 50 nm to 500 nm. The p-type region may have athickness of from 50 nm to 500 nm.

In one embodiment, the semiconductor device may be a hole-transporterfree photovoltaic device. For instance, the semiconductor device may notsubstantially comprise a layer of a p-type semiconductor. For example,in this embodiment, the semiconductor device does not comprise a layerof an organic p-type semiconductor. The semiconductor device may thus bea photovoltaic device, which photovoltaic device comprises:

-   -   an n-type region comprising at least one n-type layer;    -   a layer of an electrode material; and    -   disposed between the n-type region and the layer of an electrode        material, a layer of the semiconducting material in contact with        the layer of an electrode material.

The electrode material may be as described herein for the first orsecond electrode. For instance, the layer of the electrode material maybe a layer of a transparent conducting oxide, a layer of a metal (suchas gold) or a layer comprising graphite (for instance a carbonblack/graphite composite).

The semiconductor device may be a mesoscopic photovoltaic device, forinstance as described in Etgar, Hole-transport material-freeperovskite-based solar cells, MRS Bulletin, Vol 40, August 2015. Forinstance, the semiconductor device may comprise:

-   -   a layer of a transparent conducting oxide (for instance FTO or        ITO);    -   disposed on the layer of the transparent conducting oxide, a        layer of an n-type metal oxide (for instance TiO₂ or SnO₂),        which is optionally porous;    -   disposed on the layer of the n-type metal oxide, a layer of the        crystalline compound as defined herein (for instance in Formula        (III)); and    -   disposed on the layer of the crystalline compound, a layer of an        electrode material (for instance a metal such as gold, or an        electrode material comprising graphite).

The mesoscopic photovoltaic device may further comprise a porous layerof a dielectric material (for instance as described herein for thedielectric scaffold). This may be instead of the layer of the n-typemetal oxide in the above structure. Alternatively, the mesoscopic devicemay comprise a porous layer of an n-type metal oxide (such as TiO₂) anda porous layer of a dielectric material (such as ZrO₂).

An example of a mesoscopic cell according to the invention may comprise:

-   -   a layer of a FTO or ITO;    -   optionally, disposed on the layer of FTO or ITO, a porous layer        of ZrO₂;    -   disposed on the layer of FTO or ITO, or, if present, the porous        layer of ZrO₂, a layer of TiO₂, which is optionally porous;    -   disposed on the layer of TiO₂, a layer of the crystalline        compound as defined herein (for instance in Formula (III)); and    -   disposed on the layer of the crystalline compound, a layer of an        electrode material comprising graphite.

The semiconductor device may be a tandem photovoltaic device and furthercomprises a layer of a second semiconductor material wherein the bandgap of the second semiconductor material is lower than the band gap ofthe semiconductor material comprising the crystalline compound. Thus,the second semiconductor material and the semiconductor materialcomprising the crystalline compound may have different absorptionprofiles and absorb different parts of the solar spectrum.

The second semiconductor material may for instance comprise silicon, aperovskite, copper indium selenide (CIS), copper indium galliumdiselenide (CIGS), CdTe, PbS or PbSe. Preferably, the secondsemiconductor material comprises silicon or a perovskite. The perovskitemay be a perovskite of formula [A][B][Y]3 with [A] as one or moremonocations (for instance Cs⁺, NH₃CH₃ ⁺ or (H₂N—C(H)═NH₂)⁺), [B] as oneor more metal or metalloid dications (as defined herein, e.g. Pb²⁺) and[Y] as one or more halide anions. For instance, the perovskite may be(NH₃CH₃)PbI₃. More preferably, the second semiconductor materialcomprises silicon. The silicon is typically crystalline silicon, and maybe intrinsic silicon (i-Si), n-type silicon (n-Si) or p-type silicon(p-Si).

The tandem photovoltaic device typically comprises:

-   -   (i) a layer of silicon;    -   (ii) disposed on the layer of silicon, a layer of a transparent        conducting oxide;    -   (iii) disposed on the layer of a transparent conducting oxide,        an n-type region comprising at least one n-type layer;    -   (iv) disposed on the n-type region, a layer of said        semiconducting material;    -   (v) disposed on the layer of said semiconducting material, a        p-type region comprising at least one p-type layer; and    -   (vi) disposed on the p-type region, a layer of an electrode        material.

The invention also provides a process for producing a layer of acrystalline compound, which crystalline compound comprises: (i) Cs⁺,(ii) (H₂N—C(H)═NH₂)⁺; (iii) one or more metal or metalloid dications[B]; and (iv) a first halide anion X; and (v) a second halide anion X′,which process comprises:

(a) disposing on a substrate a precursor composition comprising:

-   -   CsX and/or CsX′;    -   (H₂N—C(H)═NH₂)X and/or (H₂N—C(H)═NH₂)X′;    -   BX₂ and/or BX′₂.

The components of the precursor composition may be disposed on thesubstrate (simultaneously or separately) by vapour deposition. Disposingon a substrate a precursor composition may therefore comprise:

-   -   (Ai) exposing the substrate to one or more vapours, which one or        more vapours comprise said precursor composition; and    -   (Aii) allowing deposition of the one or more vapours onto the        substrate to produce a layer of the crystalline compound        thereon.

The components of the precursor composition may be disposed on thesubstrate (simultaneously or separately) by solution deposition.Disposing on a substrate a precursor composition may therefore comprise:

-   -   (Bi) disposing the precursor composition and one or more        solvents on the substrate; and    -   (Bii) removing the one or more solvents to produce on the        substrate a layer of crystalline compound.

In the process of the invention, the crystalline compound may be asfurther defined herein. Preferably, the precursor composition comprises:

-   -   CsI and/or CsBr;    -   (H₂N—C(H)═NH₂)I and/or (H₂N—C(H)═NH₂)Br;    -   PbI₂;    -   PbBr₂; and    -   a polar aprotic solvent.

Examples of polar aprotic solvents include dimethylformamide (DMF),acetonitrile and dimethylsulfoxide (DMSO).

For instance, the process of the invention may comprise disposing on asubstrate a composition comprising: dimethylformamide (as a solvent);CsI and/or CsBr; (H₂N—C(H)═NH₂)I and/or (H₂N—C(H)═NH₂)Br; PbI₂; andPbBr₂; and removing the solvent. The composition may be disposed on thesubstrate by spin coating.

Removing the one or more solvents typically comprises heating the one ormore solvents or allowing the one or more solvents to evaporate. Thesubstrate, solvent or first region may be heated at a temperature offrom 40° C. to 100° C. for a time of from 5 minutes to 2 hours to removethe one or more solvents.

The substrate may be any suitable substrate. Typically the substratecomprises a layer of an electrode material and a layer of an n-typesemiconductor, for instance a compact layer of SnO₂ or TiO₂.

The invention also provides a process for producing a semiconductordevice, which process comprises a process for producing a layer of acrystalline compound as defined herein. The semiconductor deviceproduced may be as further defined herein.

After the layer of the crystalline compound is deposited, the processmay further comprise a step of annealing the layer of the crystallinecompound. For instance, the layer of the crystalline compound may beheated to a temperature of from 50° C. to 200° C., or from 70° C. to150° C. The second region may be heated to a temperature of from 90° C.to 110° C. The layer of the crystalline compound may be heated for atime from 30 seconds to 60 minutes, for instance from 2 minutes to 25minutes.

Typically, the process further comprises disposing a third region on thelayer of the crystalline compound, wherein: said third region is ap-type region comprising at least one p-type layer; or said third regionis an n-type region comprising at least one n-type layer.

The third region is typically a p-type region comprising at least onep-type layer, preferably wherein the at least one p-type layer comprisesan organic p-type semiconductor. The p-type region may be as describedabove.

The third region is typically disposed on the layer of the crystallinecompound until it has a thickness of from 50 nm to 1000 nm, for instance100 nm to 500 nm. Disposing the third region on the layer of thecrystalline compound typically comprises disposing a compositioncomprising a p-type material and a solvent on the layer of thecrystalline compound (for instance by spin-coating) and removing thesolvent. The p-type material may be any p-type material describedherein. Preferably, said third region is a p-type region comprising atleast one p-type layer, preferably wherein the at least one p-type layercomprises an organic p-type material, for instance spiro-OMeTAD.

The process typically further comprises: disposing one or more secondelectrodes on the third region. The one or more second electrodes may beas defined above for an semiconductor device according to the invention.For instance, the second electrodes may comprise a metal such as silver.The one or more second electrodes are typically disposed by vacuumvapour deposition, for instance by evaporation at a low pressure (e.gless than or equal to 10⁻⁵ mbar) optionally through a shadow mask.

The invention is now described in more detail with reference to thefollowing Examples.

EXAMPLES Example 1—Synthesis of Devices Contain (FA/Cs)Pb(I/Br)₃Perovskites Materials and Methods: Materials:

Unless otherwise stated, all materials were purchased from Sigma-Aldrichor Alfa Aesar and used as received. Spiro-OMeTAD was purchased fromBorun Chemicals and used as received.

Perovskite Precursor Synthesis:

Formamidinium iodide (FAI) and formamidinium bromide (FABr) weresynthesised by dissolving formamidinium acetate powder in a 1.5× molarexcess of 57% w/w hydroiodic acid (HI), or 48% w/w hydrobromic acid (forFABr). After addition of acid the solution was left stirring for 10minutes at 50° C. Upon drying at 100° C. for 2 h, a yellow-white powderis formed. This was then washed three times with diethyl ether. Thepower was later dissolved in ethanol heated at 80° C. to obtain asupersaturate solution. Once fully dissolved, the solution is thenplaced in a refrigerator for overnight recrystallization. Therecrystallization process forms white needle-like crystals. The powderis later washed with diethyl ether three times. Finally, the powder isdried overnight in a vacuum oven at 50° C.

Perovskite Precursor Solution Mixture:

For XRD and optical measurements, two series of films ranging from neatI to neat Br were formed; one with FA as the only cation and one with83% FA and 17% Cs. To form each specific composition ranging from neatBr to neat I, two separate precursor solutions were made: FAPbI₃ andFAPbBr₃, for the FA series. Two additional precursor solutions weremade: FA_(0.83)Cs_(0.17)PbI₃ and FA_(0.83)Cs_(0.17)PbBr₃, for the FA/Csseries. All solutions were dissolved in anhydrous N,N-dimethylformamide(DMF) to obtain a stoichiometric solution with desired composition usingprecursor salts: FAI, FABr, CsI, CsBr, PbI₂, PbBr₂. 31.7 μl of 57% w/whydroiodic acid (HI) and 18.8 μl of 48% w/w hydrobromic acid (HBr) wereadded to 1 ml of 0.55M precursor solutions for both solutions. To formthe FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ “optimized precursor solutioncomposition” used for the device fabrication, FAI, CsI, PbBr₂ and PbI₂were dissolved in DMF to obtain a stoichiometric solution with desiredcomposition and a molar concentration of 0.95M. 54.7 μl of HI and 27.3μl of HBr was added to 1 ml of 0.95M precursor solutions. After theaddition of the acids, the solution was stirred for 72 hours under anitrogen atmosphere.

Perovskite Solar Cell Fabrication:

The precursor perovskite solution was spin-coated in a nitrogen-filledglovebox at 2000 rpm for 45 s, on a substrate pre-heated at 70° C. Thefilms were dried inside a N₂ glovebox on a hot plate at a temperature of70° C. for 1 minute. The films were then annealed in an oven in an airatmosphere at 185° C. for 90 minutes.

Hole-Transporting Layer Fabrication:

The electron-blocking layer was deposited with a 96 mg/ml of2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene(spiro-OMeTAD) solution in chlorobenzene. Additives of 32 μl of lithiumbis(trifluoromethanesulfonyl)imide (170 mg/ml 1-butanol solution) per 1ml of spiro-OMeTAD solution and 10 ul of 4-tert-butylpyridine per 1 mlof spiro-OMeTAD solution. Spin-coating was carried out in anitrogen-filled glovebox at 2000 rpm for 60 s.

Electrode:

A 120 nm silver electrode was thermally evaporated under vacuum of ≈10⁻⁶Torr, at a rate of ≈0.2 nm·s⁻¹.

Device Characterization:

The current density-voltage (J-V) curves were measured (2400 SeriesSourceMeter, Keithley Instruments) under simulated AM 1.5 sunlight at100 mWcm⁻² irradiance generated by an Abet Class AAB sun 2000 simulator,with the intensity calibrated with an NREL calibrated KG5 filtered Sireference cell. The mismatch factor was calculated to be less than 1%.The active area of the solar cell is 0.0919 cm⁻². The forward J-V scanswere measured from forward bias (FB) to short circuit (SC) and thebackward scans were from short circuit to forward bias, both at a scanrate of 0.38V s⁻¹. A stabilization time of 5 s at forward bias of 1.4 Vunder illumination was done prior to scanning. The EQE was measuredusing Fourier transform photocurrent spectroscopy. The EQE was measuredin short-circuit (Jsc) configuration following a 1.4V prebias for 20 s,using a simulated air-mass (AM) 1.5 100 mW cm-2 sun light asillumination source.

Substrate Preparation:

Devices were fabricated on fluorine-doped tin oxide (FTO) coated glass(Pilkington, 7Ω□⁻¹). Initially, FTO was removed at specific regionswhere the anode contact will be deposited. This FTO etching was doneusing a 2M HCl and zinc powder. Substrates were then cleanedsequentially in hallmanex detergent, acetone, isopropyl alcohol. The FTOwas then cleaned for 10 minutes using oxygen plasma. A hole-blockinglayer was formed by immersing the cleaned FTO substrate in a bath of 40mM SnCl₄ for 30 minutes at 80° C. The substrates were rinsed in twoconsecutive deionized water baths and then sonicated for 10 s in anethanol bath. The substrates were then dried with a nitrogen gun. A 7.5mg/ml of Phenyl-C60-butyric acid methyl ester (PCBM) solution inchlorobenzene was then spin coated onto the SnO₂ compact layer at 2000rpm for 45 seconds and annealed at 70° C. for 10 minutes inside a N2glove box.

Optical Pump-THz Probe Spectroscopy:

The optical-pump-THz-probe setup uses a Spectra Physics Ti: Sapphireregenerative amplifier to generate 40 fs pulses at a center wavelengthof 800 nm and a repetition rate of 1.1 kHz. Terahertz pulses weregenerated by optical rectification in a 450 μm thick GaP(110) singlecrystal and detected by electro-optic sampling in a ZnTe crystal (0.2 mm(110)-ZnTe on 3 mm (100)-ZnTe). Pulses for optical excitation of thesamples at 400 nm have been generated using a beta barium boratfrequency doubling crystal. Optical excitation was carried out from thesubstrate side of the film. The diameters of optical pump and THz probebeams at the sample position were 3.6 mm and 2.4 mm (FWHM),respectively. Measurements were performed with the entire THz beam path(including emitter, detector and sample) in an evacuated chamber at apressure of <10⁻² bar.

Solar Cell Fabrication:

The experimental details on the silicon cell fabrication can be foundelsewhere. (33) Briefly, both-side random pyramid textured float zonen-type <100> oriented wafers with 4 inch diameter, ˜250 μm thickness,and a resistivity of 2-5 Ω·cm were used. The wafers were cleaned usingthe standard RCA process and the resulting oxides were removed bydipping in diluted hydrofluoric acid immediately before a-Si:Hdeposition. Intrinsic a-Si:H layers were deposited by standard PECVDprocesses using silane, SiH4, as precursor gas. The n-type and p-typedoped a-Si:H layers were prepared by adding PH3 or B2H6 to the precursorgas, respectively. On the front side of the wafer, 80 nm ITO wasdeposited by RF magnetron sputter deposition from a ceramic target atroom temperature. The back contact was formed by sputtering 80 nm ofaluminum doped zinc oxide (AZO) and 200 nm silver in a Leybold OpticsA600V7 tool. The front side contact grid consists of a stack of 10 nm Tiand 1500 nm Ag, thermally evaporated through a shadow mask. Followingthis fabrication process, the cells were annealed at 160° C. for 70 minin air.

Transparent Electrode:

A thin layer of ITO (Indium Tin Oxide) nanoparticles (<100 nm), from ITOdispersion in isopropanol, was spin coated on hole transport layer asthe buffer layer to protect the spiro-OMeTAD during ITO sputtering. Thena ˜120-nm thick layer of ITO was sputter coated on the buffer layerusing a PVD 75 Kurt J. Lesker.

Results and Discussion

By the above method, a highly crystalline material was produced withcomplete tunability of the band gap around 1.75 eV, and charge mobility,recombination dynamics and low electronic disorder comparable to thoseof neat lead triiodide perovskites were observed. High efficiencysolution-processed planar heterojunction solar cells were fabricatedwhich demonstrated PCEs of over 17% and stabilized power outputs (SPO)of 16%. To demonstrate the potential impact of this new perovskitematerial in tandem solar cells, an indium tin oxide (ITO) top electrodewas included to create a semi-transparent perovskite solar cell, theperformance of a silicon cell was measured after “filtering” the sunlight through the perovskite top cell. The silicon cells delivered anefficiency boost of 7.3% measured on the rear side of a semi-transparentperovskite cell, indicating the feasibility for achieving greater than25% efficient perovskite-silicon tandem cells.

In the 3D perovskite structure ABX₃, the A-site cations which could beemployed with lead halides to form suitable perovskites for solar cellapplications are typically Cs, MA and FA. CsPbI₃ does form a “blackphase” perovskite with a band gap of 1.73 eV, however, this appropriatephase is only stable at temperatures above 200 to 300° C. and the moststable phase at room temperature is a non-perovskite orthorhombic“yellow” phase, rendering it less useful. MA based perovskites arethermally unstable and suffer from halide segregation instabilities, andare thus likely to be unsuitable. FA based perovskites are the mostlikely to deliver the best balance between structural and thermalstability. However, in FIG. 1A are shown photographs of a series ofFAPb(I_((1-x))Br_(x))₃ films ranging from neat Br to neat I; a“yellowing” of the films is observed for compositions between x=0.3 andx=0.6 of the FAPb(I_((1-x))Br_(x))₃ system, consistent with thepreviously reported phase instability due to a transition from atrigonal (x<0.3) to cubic (x>0.5) structures. It has previously beenobserved that the band gap changes from 1.48 eV for FAPbI₃ to 1.73 eVfor CsPbI₃. The inventors therefore considered the possibility that ifwe partially substitute FA for Cs, we may be able to push this region ofstructural instability to higher energies, and thus achieve astructurally stable mixed halide perovskite with a band gap of 1.75 eV.In FIG. 1B are shown photographs of thin films fabricated frommixed-cation lead mixed-halide FA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃compositions. Unexpectedly, the region of structural instability is notsimply shifted to higher energy, but a continuous series of dark filmsis observed throughout this entire Br to I compositional range. Toconfirm these observations, also performed were UV-vis absorptionmeasurements. As shown in FIG. 2 (top) and (bottom), a sharp opticalband edge for all Br—I compositions of the mixed-cationFA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃ perovskite material wereobtained, in contrast to FAPb(I_((1-x))Br_(x))₃ which shows weakabsorption in the intermediate range.

In order to understand the impact of adding caesium upon thecrystallization of the perovskite in more detail, X-ray diffraction(XRD) studies were performed on the series of films covering the I to Brcompositional range. In FIG. 3 (top) is shown the XRD patters forFAPb(I_((1-x))Br_(x))₃, zoomed in on the peak around 2θ˜14°.FAPb(I_((1-x))Br_((x)))₃ perovskites were formed on fluorine-doped tinoxide (FTO) coated glass substrates when annealed at 170° C. for 10 m,using a 0.55M solution. The complete diffraction pattern is shown inFIG. 12. FAPb(I_((1-x))Br_(x))₃ undergoes a structural phase transitionfrom a trigonal phase for x<0.3 into a cubic phase for x>0.5, withstructural instability in the intermediate region. Quite remarkably, forthe FA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃ the material is in a singlephase throughout the entire compositional range (see FIG. 1F). Themonotonic shift of the (100) reflection which we observe from 2θ˜14.2°to 14.9° is consistent with a cubic lattice constant shift from 6.306 Åto 5.955 Å as the material incorporates a larger fraction of the smallerhalide, Br. The complete diffraction pattern is shown in FIG. 13.Therefore, it can be confidently stated that for theFA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃ perovskite, the structural phasetransition has been removed and stability is now present over the entirecompositional range. The impact of varying the Cs concentration and theBr to I concentration are shown in FIGS. 14 to 20. Quite remarkably overthe entire Br to I range, and for a large fraction of the Cs-FA rangethe variation in lattice constant, composition and band gap preciselyfollow Vegard's law, as shown in FIG. 21. This indicates a totalflexibility and predictability in tuning of the composition and itsimpact upon the band gap. For the remainder of Example 1, the precisecomposition FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ which has a band gapof 1.74 eV (FIG. 22) is used unless stated otherwise.

Photo-induced halide segregation has been reported in methylammoniumlead mixed-halide perovskites. A red-shift in PL upon lightillumination, with intensities ranging from 10-100 mW cm⁻² occurs, withthe shift to lower energies resulting from the formation of iodine richdomains which have lower band gaps. This limits the achievableopen-circuit voltage of the solar cell device by introducing a largedegree of electronic disorder. In FIGS. 4A and 4B the photoluminescencefrom films of MAPb(I_(0.6)Br_(0.4))₃ perovskite and the mixed cationmixed halide material FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ are shown.The PL from both films was measured immediately after prolonged periodsof light exposure, using a power density of 3 mW cm⁻² and a wavelengthof 550 nm as excitation source. The halide segregation results observedby Hoke et al. were confirmed where we see a significant time-dependentred-shift in PL for the MAPb(I_(0.6)Br_(0.4))₃ film, which exhibits a130 meV PL red-shift after only 1 hour of illumination. However,although a rise in PL intensity is seen, no significant red-shift in PLemission for the FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ precursorcomposition is observed after 1 hour of identical light illumination(which is shown in FIG. 5). Furthermore, as shown in FIG. 23, we alsoexposed a similar FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ film tomonochromatic irradiance of much higher 5 Wcm⁻² irradiance, and observeno red shift after 240 seconds of illumination. Under these identicalconditions, a red shift is observed in the PL for the single cationFAPb(I_(0.6)Br_(0.4))₃ perovskite.

Beyond halide segregation, a further deleterious observation previouslymade for mixed halide perovskites has been that the energetic disorderin the material is greatly increased in comparison to the neat iodideperovskites. The ultimate open-circuit voltage a solar cell material cangenerate is intimately linked to the steepness of the absorption onsetjust below the band edge, which can be quantified by a term known as theUrbach energy (E_(u)). This E_(u) reported for MAPbI₃ was 15 meV, wheresmall values of E_(t), indicate low levels of electronic disorder. Incontrast, the E_(t), for MAPb(I_(0.6)Br_(0.4))₃ perovskite increases to49.5 meV which represents similar levels of electronic disorder asorganic photovoltaics and amorphous silicon. In order to determine theUrbach energy for the present system, Fourier transform photocurrentspectroscopy (FTPS) is performed on complete planar heterojunction solarcells and FIG. 5 shows the semi-log plot of external quantum efficiency(EQE) absorption edge of a device fabricated with the optimizedprecursor solution and annealing procedure. From this measurement, anUrbach energy (E_(u)) of 16.5 meV may be calculated very close to thevalues reported for the neat iodide perovskites. This shows that thecrystalline compound of the invention has very favourable electrondisorder.

In order to further asses the electronic quantity ofFA_(0.83)Cs_(0.17)Pb(I₀₆Br_(0.4))₃ the inventors have performed opticalpump THz-probe (OPTP) spectroscopy, which is a non-contact method ofprobing the photoinduced conductivity and effective charge carriermobility in the material. FIG. 6 shows the fluence-dependence of theOPTP transients, which exhibit accelerated decay dynamics at higherinitial photoinjected charge-carrier densities, as the result ofenhanced contributions from bimolecular and Auger recombination effects.The rate constants associated with different recombination mechanismsmay be extracted by global fits to these transient of the solutions tothe rate equation:

$\frac{{dn}(t)}{dt} = {{{- k_{3}}n^{3}} - {k_{2}n^{2}} - {k_{1}n}}$

Here, k₁ is the monomolecular recombination rate associated withtrap-related recombination, k₂ is the bi-molecular, and k₃ the Augerrecombination rate constant. Since the monomolecular lifetime τ=k₁ ⁻¹ issignificantly longer (156 ns) than the 2.5 ns observation window of theOPTP measurements, k₁ is determined from monoexponential fits to thetail of the photoluminescence decay transient, as shown in FIG. 24. Inaddition, one is able to determine a value for the effectivecharge-carrier mobility from the initial value of the OPTP signal underknowledge of the absorbed photon density profile. In the absence ofexcitonic effects, this value approaches the sum of charge-carriermobilities μ for electrons and holes.

It has been found that FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ exhibitsan excellent charge-carrier mobility of 21 cm²/(Vs) which comes close tothe value of ˜30 cm²/(Vs) typically found for high-quality single-halideFAPbI₃ and MAPbI₃ thin films at room temperature. This value isstriking, because it has recently been shown that the correspondingneat-FA perovskite FAPb(I_(0.6)Br_(0.4))₃ only sustains very lowcharge-carrier mobilities <1 cm²/(Vs) that are related to the amorphousand energetically disordered nature of these materials within the regionof the trigonal to cubic phase transition. Conversely,FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ examined here displays a mobilityvalue intermediate to those we have previously determined for FAPbI₃ (27cm² V⁻¹ s⁻¹) and FAPbBr₃ (14 cm² V⁻¹ s⁻¹) suggesting that it is nolonger limited by structural disorder. The inventors further assessedthe potential of FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ forincorporation into planar heterojunction PV architectures by derivingthe charge-carrier diffusion length L=(μkT/(eR))^(0.5) as function ofthe charge-carrier density n, where R=k₁+nk₂+n²k₃ is the totalrecombination rate, k the Boltzmann constant, T temperature and e theelementary charge. FIG. 7 shows that for charge-carrier densitiestypical under solar illumination (n˜10¹⁵ cm⁻³) a value of L˜2.9 μm isreached, which is comparable to values reported for high-quality thinfilms of neat lead iodide perovskites. The high charge-carrier mobilityand slow recombination kinetics, and long charge carrier diffusionlength imply that this mixed cation, mixed halide perovskite should bejust as effective as a high quality solar cell absorber material as theneat halide perovskite FAPbI₃.

A series of planar heterojunction solar cells were fabricated to assessthe overall solar cell performance. Data for solar cells fabricated witha range of compositional and processing parameters are shown in FIGS. 25and 26. The device architecture is shown in FIG. 8, which is composed ofa SnO₂/Phenyl-C60-butyric acid methyl ester (PC₆₀BM) electron selectivelayer, a solid FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ perovskiteabsorber layer, and Li-TFSI-doped Spiro-OMeTAD with 4-tert-Butylpyridine(TBP) additive as the hole-collection layer, caped with an Ag electrode.Current-voltage characteristics of such devices under simulated air-mass(AM) 1.5 100 mW cm⁻² sun light were measured, and the current-voltagecharacteristics of one the highest performing device is shown in FIG. 9.The device delivers a short-circuit current density of 19.4 mA cm⁻², anopen-circuit voltage of 1.2 V, and a PCE of 17.1%. By holding the cellat a fixed maximum power point forward bias voltage of 0.95V the poweroutput over time was measured as reaching a stabilized efficiency of16%, which is shown in FIG. 10. The highest JV efficiency we measuredwas 17.9%, which is shown in FIGS. 27 and 28. To demonstrate that thesecells can also operate with larger area, 0.715 cm² active layers werefabricated where the cells reach a stabilized power output of over 14%,as shown in FIGS. 29 and 30. FIG. 11 shows the spectral response of thesolar cell, which confirms the wider band gap of the solar cell, andalso integrates over the AM1.5 solar spectrum to give 19.2 mAcm⁻², inclose agreement to the measured J_(SC) of the solar cell.

This performance is very competitive with the best reported singlejunction perovskite solar cell reported so far, especially consideringthe wider band gap of compound of the invention which should result in afew % absolute efficiency drop with respect to a 1.55 eV material.Importantly for tandem solar cells, the 1.74 eV material appears to becapable of generating a higher open-circuit voltage than the 1.55 eVtri-iodide perovskites in planar heterojunction solar cells. The maximumattainable V_(OC) for a solar cell absorber material is shown in FIG.31. For the FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ device, a maximumV_(OC) of 1.42 V is estimated which is 100 mV higher than that estimatedfor MAPbI₃ devices. The crystalline material of the invention will thusdeliver a higher voltage. It is also notable that with 1.2V V_(OC), aloss of around 220 mV is still present as compared to the predictedV_(OC) in the radiative limit, indicating much scope for furtherimprovement by better selection and optimization of contact materialsand inhibition of non-radiative decay channels from within theperovskite.

In order to demonstrate the potential impact of employing this newperovskite composition in a tandem architecture, semitransparentperovskite solar cells were fabricated by sputter coating ITO on top ofthe perovskite cells, with the additional inclusion of a thin “bufferlayer” of solution processed ITO nanoparticles between the spiro-OMeTADand the ITO in order to inhibit sputter damage to the perovskite cell.The efficiency of the semi-transparentFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ solar cells was 15.1%, asdetermined by the current voltage curve, with a stabilized power outputof 12.5%. Since the J_(SC) is very similar to the cell with the Agelectrode, it is expected that the slight drop in V_(OC) and SPO will besurmountable by better optimization of the ITO sputter depositionprocedure. A silicon heterojunction (SHJ) cell was measured with andwithout a semi-transparent perovskite cell held in front of it, and anefficiency of 7.3% filtered was determined and 19.2% when uncovered, asshown in FIG. 32. These results demonstrate the feasibility of obtaininga combined tandem solar cell efficiency ranging from 19.8% (if they arecombined with the stabilized power output of the semi-transparent cell)to 25.2% (if they are combined with the highest JV measured efficiencyof the FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ cell).

In summary, the inventors have tailored a perovskite composition to beperfectly suited for incorporation into silicon tandem solar cells. Theyhave addressed the critical issues that were limiting the use ofperovskite as a viable top-cell material for tandem applications, anddelivered a thermally, structurally and compositionally stable materialof the correct band gap. Surprisingly, theseFA_(0.83)Cs_(0.17)Pb(I₀₆Br_(0.4))₃ films exhibit a very steep rise inabsorption and EQE onset below the band gap, an excellent charge-carriermobility of 21 cm²/(Vs) and diffusion length of 3 μm, all of whichindicate that is electronically homogeneous with low energetic disorderand as high a quality semiconductor as the tri-iodide perovskites.Moreover, the inventors have fabricated solar cells reaching highvoltages in excess of 1.2V and achieving over 17% power conversionefficiency and an SPO of 16% in single junctions. They have additionallydemonstrated the feasibility of creating perovskite silicon tandem solarcells with efficiencies ranging from 19.8 to 25.2%. Considering furtherminor improvements in the perovskite, optical management and integrationand choice of silicon rear cell, it is feasible that this system coulddeliver up to 30% efficiency in the near future. In addition, thismonotonic tunability of band gap across the visible spectrum within asingle crystalline phase, will have direct impact to the colortunability and optimization of perovskites for light emittingapplications.

Example 2—Variation of Br Content

Devices containing various perovskites of the formulaFA_(0.83)Cs_(0.17)Pb(I_((1-x))Br_(x))₃ were synthesis by a methodequivalent to that described in Example 1. The PCE of each device wasevaluated as for Example 1 and the results are shown in FIG. 33. Thedevices generally perform well with different bromine contents, and animprovement is observed for contents of over 10%.

Example 3—Electroluminescence Studies

Examples 1 and 2 concern the photoabsorbent properties of the mixedcation/mixed halide perovskites of the invention. The crystallinecompounds of the invention have also been found to be effectivephotoemissive compounds. As such, light emitting devices could beconstructed containing the crystalline compounds of the invention.

In particular, a series of FA_(0.825)Cs_(0.175)Pb(I_((1-x))Br_(x))₃ withx=[0, 0.05, 0.1, 0.2, 0.3, 0.4] devices was prepared following the samefabrication method described for photovoltaic devices. To measure theelectroluminescence intensity, the devices were kept in a chamber filledwith Nitrogen and placed inside an integrating sphere. The emitted lightis collected with a fiber and analysed with a fixed grating CCDspectrometer (Maya Pro, Ocean Optics). The intensity of the luminescenceis estimated from the area of the luminescence peak. The current-voltage(IV) properties are measured simultaneously with a Keithley sourcemeter(Model 2600).

FIG. 34 shows the normalized electroluminescence spectra of the seriesof FA_(0.825)Cs_(0.175)Pb(I_((1-x))Br_(x))₃ devices with x=[0, 0.05,0.1, 0.2, 0.3, 0.4]. FIG. 35 shows the electroluminescence spectra ofFA_(0.825)Cs_(0.175)Pb(I_((1-x))Br_(x))₃ with x=0.3 as a function ofapplied voltage. The plots in FIG. 36 show IV and LV performance for theseries of FA_(0.825)Cs_(0.175)Pb(I_((1-x))Br_(x))₃ with x=[0, 0.05, 0.1,0.2, 0.3, 0.4]. The cell area was 0.909 cm².

Example 4—Comparative Stability Study

In order to assess the long term operational stability ofFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ solar cells, stabilitymeasurements were performed for solar cells comprising the mixed-cationlead mixed-halide perovskite as the absorber layer. The stability ofsolar cells comprising the mixed-cation perovskite according to theinvention were compared with reference devices comprising themethylammonium lead halide perovskite MAPbI_(x)Cl_(3-x) as the absorber.

The devices comprised a layer of C₆₀ disposed on a SnO₂ electrontransport layer. The C₆₀ layer was doped or undoped with N-DPBI(dihydro-1H-benzoimidazol-2-yl). The device configuration wasFTO/SnO₂/C₆₀ (neat or 1 wt % doped)/perovskite/spiro-OMeTAD(doped withLi-TFSI and tBP)/Au. The devices were aged under full spectrum simulatedAM1.5, 76 mWcm⁻² average irradiance at V_(OC) in air without a UVfilter.

The results for the cells light soaked (AM1.5 full spectrum light) atV_(OC) in air with ambient humidity (˜55%) are shown in FIG. 37. Thecells in FIG. 37(a) are not encapsulated. The cells comprising thisFA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ perovskite exhibits a muchstronger resistance to degradation under the aging conditions tested.All cells exhibit a fast degradation over the first 50 hrs, with theMAPbI_(x)Cl_(3-x) cell degrading to approximately zero % efficiency overthis time. In contrast, the FA_(0.83)Cs_(0.17)Pb(I_(0.6)Br_(0.4))₃ cellsonly degrade by a few % absolute efficiency over this time, and thenproceed to degrade at a much slower linear rate. The stability ofdevices comprising the mixed-cation perovskite according to theinvention which have been encapsulated with a hot-melt polymer foil andglass cover-slip are shown in FIG. 37(b). These results show that themixed-cation perovskite is much more stable in general than theMAPbI_(x)Cl_(3-x) perovskite. This is both thermal and moisturestability, and importantly stability to operation in the presence ofoxygen.

Example 5—Absorbance, Luminescence and XRD Studies of Mixed-CationPerovskites

Layers of FA_(0.83)Cs_(0.17)Pb(Cl_(X)Br_(Y)I_(Z))₃ perovskites wereformed on fluorine-doped tin oxide (FTO) coated glass substrates andannealed at 150° C. for 30 min in nitrogen, using a 0.1M solutiondissolved in a 4:1 DMF:DMSO solvent mixture and a nitrogen flow toquench crystallization during spin-coating.

The UV-vis absorbance spectra of the perovskites were measured and theresults are shown in FIG. 38.

The normalized photoluminescence spectra for the visible range of theperovskites were measured and the results are shown in FIG. 39.Excitation wavelength varied from 375 nm to 600 nm.

X-ray diffraction patterns (XRD) of the perovskites were measured andthe results are shown in FIG. 40.

1. A crystalline compound comprising: (i) Cs⁺, (ii) (H₂N—C(H)═NH₂)⁺;(iii) one or more metal or metalloid dications [B]; and (iv) two or moredifferent halide anions [X].
 2. A crystalline compound according toclaim 1, wherein the two or more different halide anions [X] areselected from I⁻, Br⁻ and Cl⁻.
 3. A crystalline compound according toany claim 1 or claim 2, wherein the two or more different halide anions[X] are I⁻ and Br⁻.
 4. A crystalline compound according to any one ofclaims 1 to 3, wherein the one or more metal or metalloid dications [B]are selected from Pb²⁺, Sn²⁺, Ge²⁺ and Cu²⁺.
 5. A crystalline compoundaccording to any one of the preceding claims, wherein the one or moremetal or metalloid dications [B] are Pb²⁺.
 6. A crystalline compoundaccording to any one of the preceding claims, wherein the crystallinecompound is a perovskite compound of formula (I):Cs_(x)(H₂N—C(H)═NH₂)_((1-x))[B][X]₃  (I); wherein: [B] is the one ormore metal or metalloid dications; [X] is the two or more differenthalide anions; and x is from 0.01 to 0.99.
 7. A crystalline compoundaccording to any one of the preceding claims, wherein the crystallinecompound is a perovskite compound of formula (II):Cs_(x)(H₂N—C(H)═NH₂)_((1-x))[B]X_(3y)X′_(3(1-y))  (II); wherein: [B] isthe one or more metal or metalloid dications; X is a first halide anionselected from I⁻, Br⁻, Cl⁻ and F⁻; X′ is a second halide anion which isdifferent from the first halide anion and is selected from I⁻, Br⁻, Cl⁻and F⁻; x is from 0.01 to 0.99; and y is from 0.01 to 0.99.
 8. Acrystalline compound according to claim 7, wherein X is Br⁻ and X′ isI⁻.
 9. A crystalline compound according to any one of claims 6 to 8,wherein x is from 0.05 to 0.50, preferably wherein x is from 0.10 to0.30.
 10. A crystalline compound according to any one of claims 6 to 9,wherein x is from 0.15 to 0.20.
 11. A crystalline compound according toany one of claims 6 to 10, wherein y is from 0.01 to 0.70, preferablywherein y is from 0.20 to 0.60.
 12. A crystalline compound according toany one of claims 6 to 11, wherein y is from 0.30 to 0.50.
 13. Acrystalline compound according to any one of the preceding claims,wherein the crystalline compound is a perovskite compound of formula(III):Cs_(x)(H₂N—C(H)═NH₂)_((1-x))PbBr_(3y)I_(3(1-y))  (III); wherein: x isfrom 0.15 to 0.20; and y is from 0.30 to 0.50.
 14. A crystallinecompound according to any one of the preceding claims, wherein thecrystalline compound isCs_(0.175)(H₂N—C(H)NH₂)_(0.825)Pb(Br_(0.4)I_(0.6))₃.
 15. A crystallinecompound according to any one of the preceding claims, wherein thecrystalline compound has a band gap of from 1.5 to 2.0 eV.
 16. Acrystalline compound according to any one of the preceding claims,wherein the crystalline compound is in the form of particles comprisingthe crystalline compound.
 17. A semiconducting material comprising acrystalline compound as defined in any one of claims 1 to 16, preferablywherein the semiconducting material is a photoactive material.
 18. Asemiconducting material according to claim 17, wherein thesemiconducting material comprises greater than or equal to 80% by weightof the crystalline compound.
 19. A semiconductor device comprising asemiconducting material, which semiconducting material comprises acrystalline compound, which crystalline compound comprises: (i) Cs⁺,(ii) (H₂N—C(H)═NH₂)⁺; (iii) one or more metal or metalloid dications[B]; and (iv) two or more different halide anions [X].
 20. Asemiconductor device according to claim 19, wherein the crystallinecompound is as further defined in any one of claims 2 to
 16. 21. Asemiconductor device according to claim 19 or claim 20, wherein thesemiconductor device is an optoelectronic device, preferably wherein thesemiconductor device is a photovoltaic device, a photodetector or alight-emitting device, more preferably wherein the semiconductor deviceis a photovoltaic device.
 22. A semiconductor device according to anyone claims 19 to 21, which semiconductor device comprises a layer ofsaid semiconducting material, preferably wherein the layer of saidsemiconducting material has a thickness of from 5 nm to 10000 nm.
 23. Asemiconductor device according to any one of claims 19 to 22, whichsemiconductor device comprises: an n-type region comprising at least onen-type layer; a p-type region comprising at least one p-type layer; and,disposed between the n-type region and the p-type region: a layer ofsaid semiconducting material.
 24. A semiconductor device according toany one of claims 19 to 23, which semiconductor device is a tandemphotovoltaic device and further comprises a layer of a secondsemiconductor material wherein the band gap of the second semiconductormaterial is lower than the band gap of the semiconductor materialcomprising the crystalline compound, preferably wherein the secondsemiconductor material comprises silicon, a perovskite, copper indiumselenide (CIS), copper indium gallium diselenide (CIGS), CdTe, PbS orPbSe.
 25. A semiconductor device which is a tandem photovoltaic deviceaccording to claim 24, wherein the tandem photovoltaic device comprises:(i) a layer of silicon; (ii) disposed on the layer of silicon, a layerof a transparent conducting oxide; (iii) disposed on the layer of atransparent conducting oxide, an n-type region comprising at least onen-type layer; (iv) disposed on the n-type region, a layer of saidsemiconducting material; (v) disposed on the layer of saidsemiconducting material, a p-type region comprising at least one p-typelayer; and (vi) disposed on the p-type region, a layer of an electrodematerial.
 26. A semiconductor device according to claim 19 or claim 20,which device comprises a layer of a transparent conducting oxide;disposed on the layer of the transparent conducting oxide, a layer of ann-type metal oxide; disposed on the layer of the n-type metal oxide, alayer of said crystalline compound; and disposed on the layer of thecrystalline compound, a layer of an electrode material.
 27. A processfor producing a layer of a crystalline compound, which crystallinecompound comprises: (i) Cs⁺; (ii) (H₂N—C(H)═NH₂)⁺; (iii) one or moremetal or metalloid dications [B]; and (iv) a first halide anion X; and(v) a second halide anion X′, which process comprises: (a) disposing ona substrate a precursor composition comprising: CsX and/or CsX′;(H₂N—C(H)═NH₂)X and/or (H₂N—C(H)═NH₂)X′; BX₂ and/or BX′₂.
 28. A processaccording to claim 27, wherein disposing on a substrate a precursorcomposition comprises: (Ai) exposing the substrate to one or morevapours, which one or more vapours comprise said precursor composition;and (Aii) allowing deposition of the one or more vapours onto thesubstrate to produce a layer of the crystalline compound thereon; or(Bi) disposing the precursor composition and one or more solvents on thesubstrate; and (Bii) removing the one or more solvents to produce on thesubstrate a layer of crystalline compound.
 29. A process according toclaim 27 or claim 28, wherein the crystalline compound is as furtherdefined in any one of claims 1 to
 16. 30. A process according to any oneof claims 27 to 29, wherein the precursor composition comprises: CsIand/or CsBr; (H₂N—C(H)═NH₂)I and/or (H₂N—C(H)═NH₂)Br; PbI₂; PbBr₂; and apolar aprotic solvent.
 31. A process for producing a semiconductordevice, which process comprises a process for producing a layer of acrystalline compound as defined in any one of claims 27 to
 30. 32. Aprocess according to claim 31, wherein the semiconductor device is asfurther defined in any one of claims 19 to 26.